Normally NOx and other harmful substances are contained in exhaust gas from internal combustion engines. A NOx catalyst is provided in the exhaust system of the internal combustion engine in order to purify NOx in the exhaust. In the case of a storage-reduction type NOx catalyst used for this purpose, however, purification performance decreases as the amount of NOx stored increases. As a countermeasure, fuel that acts as a reducer is supplied to the storage-reduction type NOx catalyst so as to reduce and release NOx stored in the catalyst (hereinafter referred to as a “NOx reduction treatment”). It is also common knowledge that SOx in exhaust that is stored in the storage-reduction type NOx catalyst results in decreased purification performance, due to so-called SOx poisoning. Fuel that acts as a reducer is sometimes supplied to the storage-reduction type NOx catalyst in order to counteract SOx poisoning (hereinafter referred to as a “SOx poisoning recovery treatment”).
When supplying fuel that acts as a reducer to the storage-reduction type NOx catalyst, however, some of the supplied fuel may not be fully used in the oxidation reaction within the storage-reduction type NOx catalyst, if the amount of exhaust flow introduced to the storage-reduction type NOx catalyst is not appropriate. This means that the NOx reduction treatment and the SOx poisoning recovery treatment may not be adequately performed, and fuel efficiency may also decrease.
In light of this, art has been proposed that, in order to reduce the amount of accumulated NOx in the NOx catalyst, at first reduces the amount of exhaust gas flowing to the NOx catalyst, and next supplies a reducer when a preset time has passed after the reduction. Furthermore, art has been proposed that uses an oxygen concentration sensor to detect an oxygen concentration in exhaust gas discharged from the NOx catalyst when a reducer is supplied. A high purification rate is thus achieved by correcting the above-mentioned preset time so that the peak value of the oxygen concentration coincides with a target value (see Japanese Patent Application Publication No. JP-A-2004-52603 for an example).
However, the dispersibility of the added fuel remains poor in the above cases, and the fuel may tend to concentrate at a portion of the NOx catalyst. This tendency becomes greater in cases where the NOx catalyst has a long length, or a plurality of NOx catalysts is arranged in series. Even when the fuel-adding timing is controlled based only on the state quantity of exhaust gas downstream of the NOx catalyst, there is a limit in such cases as to how much the dispersibility of the added fuel in the NOx catalyst can be raised. More improvements can still be made to ensure that the NOx reduction treatment is sufficiently performed throughout the entire NOx catalyst. On a side note, related art is also described in Japanese Patent Application Publication No. JP-A-H10-121944, Japanese Patent Application Publication No. JP-A-2000-230420, and Japanese Patent Application Publication No. JP-A-2003-214150.